Bagni di Tivoli Travertine:
The
white stone that you see all over Rome, from the fountains to
the old noble
palazzos and from St. Peter's church to the Colosseum, is
almost always
travertine rather than marble. Although travertine looks
porous, with its
gaps, bubbles, grain, and inclusions, the stone actually is
much less permeable
than marbles, limestones, and sandstones. It is not as strong
and impermeable
as granites or basalts, but it is much cheaper because it is
more easily
quarried and worked and much more available. It has been the
structural
stone of choice in Rome for 2000 years because it is stronger
than marble
and because one of the largest deposits of good travertine in
the world
is just twenty miles down the road at the base of the
Tiburtine mountains.

Travertine is a product of
the earth's
water and carbon cycles. As carbon dioxide-rich rainwater
percolates through
soil and stone, it slowly dissolves tremendous quantities of
limestone
along underground fissures. This water, now saturated with
dissolved limestone,
reemerges at the surface at warm or hot springs and releases
carbon dioxide
gas into the atmosphere -- much like carbonated mineral
water. Once the
carbon dioxide is released, the limestone can no longer
remain in solution.
It re-crystallizes, typically as the water cascades over
organic films
made of bacteria, algae, and mosses. A dense, banded
carbonate stone is
built up over time as new material covers older layers. This
process has
continued at the Bagni di Tivoli springs for at least 80,000
years. (This
is the same process that builds up the terraces around the
springs at Yellowstone
National Park in the United States. The "yellow stone" there
is fresh sulfurous
travertine of almost the exact same chemical composition as
Bagni di Tivoli
stone.)

Calcite and gypsum, the
minerals
that make up about 99 percent of travertine stone, are
colorless. The beautiful
honey color of Tivoli travertine is from the other 1 percent
of the stone:
traces of yellow sulfur, brown iron compounds, and organic
pigments. The
intricate "Swiss cheese" texture of travertine is partly the
result of
gas bubbles, which are often trapped between layers of
stone, creating
spherical voids. Minerals crystallizing on the ever-present
bacteria in
travertine deposits -- like granular snow blanketing a
miniature landscape
-- preserve organic growth forms, called "shrubs," and
produce much of
the rugged relief evident if the stone is split for use
along its "grain".
In some cases, travertine layers are similar to tree rings,
with lighter
and darker laminations representing seasons of growth.

Travertine is found in
greatest
abundance where hot and cold springs have been active for
tens of thousands
of years. The most famous travertine location, is Bagni di
Tivoli, 20 kilometers
east of Rome, where travertine deposits over 90 meters thick
have been
quarried for over two thousand years. The name of the stone
is, in fact,
derived from the ancient Roman name of Tivoli, which was
Tibur. "Tibur
stone" in Latin was "lapis Tiburtinus", which with only a
little corruption
became "Travertine".

Because travertine is
plentiful,
weighs less than marble or granite, and is relatively easy
to quarry, it
was the stone most commonly used by the ancient Romans.
Famous structures
constructed with Tivoli travertine include the Colosseum,
the Trevi Fountain,
the façade and colonnade of Saint Peter's Basilica,
and many Roman
aqueducts. In the last century, Lincoln Center in New York
and the ABC
Entertainment Center in Los Angeles were faced with
travertine from the
same Tivoli quarries.

Travertine is split with
the grain
of the stone, making visible many more fossils than are seen
in the more
common banded travertine, which is cross-sectioned and
polished. At least
two species of fossilized leaves are fairly common in stone
from Bagni
di Tivoli -- evidence of a lakeside environment at the time
of the stone's
formation. Fossilized animal material is less common, but an
occasional
feather or bone is discovered. The rapid deposition of the
travertine layers
acts as a natural preservative for these traces of
prehistoric life.

In order to remove the
travertine
from the vertical quarry face, workers drill holes into the
stone, outlining
a block 6 meters high, 12 meters wide, and 2 meters deep. A
diamond-studded
cable is then threaded through the holes, lubricated with
water to prevent
heat buildup, and pulled against the stone with a set of
pulleys. A large
cut may take a day and a half, but eventually diamond wins
out over the
softer travertine. When the cuts are completed, the slab is
pushed away
from the quarry wall and falls onto mounds of earth, which
help cushion
the fall. The slab is then broken up into more manageable
cubes, which
are taken to factories for honing and cutting or splitting.
An automated
guillotine has been invented to split the stone along its
natural bedding
plane, or diamond saws can be used to cut standard or
specially ordered
cross-cut stones and slabs.

Travertine floor surfaces
are anticipated
to last at least 50 years before replacement or refinishing
is needed in
high-traffic areas. High-quality travertine is very durable,
since it is
formed at the earth's surface in relative equilibrium with
the environment.
Most other building stones are formed under different
conditions, deep
underground and may be less stable in surface temperature
and pressure
conditions.

In modern installations,
travertine
blocks may be treated with silicon-based water repellents
that are expected
to ease cleaning. Paving stone and lower parts of walls in
public areas
may also be treated with oil and paint resistant coatings to
reduce the
effects of soiling and graffiti. It's important, however,
not to completely
seal surfaces since that could cause moisture to accumulate
inside the
stone or to migrate to inner walls where it could damage
plaster or stucco
surfaces or frescoes.

Over time, the honey color
of fresh
Bagni di Tivoli travertine will change as the stone weathers
and natural
patinas forms. We, of course, are more familiar with the
"un-natural" grimy
patina on those buildings that did not get cleaned for
Rome's Jubilee Year.